The present invention relates to phase locked loop (xe2x80x9cPLLxe2x80x9d) circuits and clock sources, and more particularly to ring oscillators for high-speed communications.
Recently, the world has witnessed a phenomenal growth both in the number of web users, applications and devices, and in the amount of data traffic, especially that of medium-rich contentxe2x80x94all demanding high speed communications and connectivity over the global information network. To accommodate this surge in bandwidth demand, network carriers have begun to provide ultra high rate fiber optical channels. While data transmission can be done using fiber optical channels, the processing of such data still involves the handling of electrical signals.
In order to produce clock signals for processing data, conventional LC oscillators have been used. It has been well known that while a conventional LC oscillator may produce signals which have low noise and high quality, it suffers from having a narrow tuning range. A simplified diagram of a conventional LC oscillator is illustrated in FIG. 1. As shown, LC oscillator 50 includes resistor 520, capacitor 530, inductor 540 and its sustaining circuit 510. To be able to change the frequency, capacitor 530 is implemented by a varactor, which is a variable capacitor. The frequency, f, of LC oscillator 50 can be calculated by: f=1/squareroot(LC).
Another type of a conventional oscillator is a ring oscillator 60, a simplified diagram of which is shown in FIG. 2. It has been known that while these ring oscillators can achieve a wide tuning rage, they produce high noise. As shown, ring oscillator 60 has many stages and a number of inversions. The frequency of ring oscillator 60 is represented by: f=1/(2 Nxc2x7Td), where xe2x80x9cNxe2x80x9d is the number of stages, and xe2x80x9cTdxe2x80x9d is the delay through the stage. As an example, to achieve a frequency of 20 GHz, each stage of ring oscillator 60 should have a delay of 6.25 ps, based on: 1/(2xc2x74xc2x76.25 ps)=20 GHz.
While a conventional oscillator may work in low clock rate applications, it does not work well in high clock rate (e.g., 10 GHz or above) applications used for modem communication applications. One of the reasons for this drawback lies in the varactor component of the oscillator. A varactor, which is a variable capacitor, allows the frequency of a typical oscillator, such as an inductor/capacitor (xe2x80x9cLCxe2x80x9d) oscillator, to change based on the formula: f=1/squareroot (LC). When the capacitance is changed, the frequency of the oscillator can be changed as well. However, a varactor in a conventional oscillator has a small range such that it limits the range of the oscillator""s frequency. Additionally, at high frequencies, the parasitic capacitance of the oscillator""s interconnects and devices play a much larger role, percentage-wise, and tend to reduce the effective capacitive range of the varactors. In addition, a varactor can be noisy and lossy.
The limitation imposed by the varactor makes the manufacturing of oscillators a challenge, since its frequency range cannot be kept consistent, especially when they are in volume production. The lack of consistency, which produces a poor yield, makes it undesirable for those skilled in the art to include a varactor in an oscillator for high clock rate applications.
Some have attempted to solve the problems brought on by the varactors by using multiple oscillators on the chip to cover different ranges of frequency. The rationale was that since the varactors tend to shift together, one of the oscillators will have the desired range and can be pickedxe2x80x94on the fly. As can be appreciated by those skilled in the art, such a solution has become more complicated than the problem it is intended to solve.
Therefore, it is desirable to be able to achieve wide frequency ranges for an oscillator for high clock rate applications in a consistent manner.
It is also desirable to be able to consistently achieve wide frequency ranges for an oscillator without a varactor for high clock rate applications.
A quadrature ring oscillator for high clock-rate applications is disclosed. In accordance with one embodiment of the present invention, a quadrature LC ring oscillator of the present invention may use two stages of LC oscillators and variable mixers to provide consistent oscillation even at high clock rates. One stage of the quadrature LC ring oscillator comprises a first resonating element having an input and an output, and a first variable summer having L and P inputs and an output, with its L input being connected to the output of the first resonating element. The output of the first variable summer is connected to the input of the first resonating element. The first variable summer is adapted to generate its output at a first phase by combining the L and P inputs. A second stage of the LC ring oscillator comprises a second resonating element, which has an input and an output, with its output being connected to the P input of the first variable summer. An inverter is used to produce an inverted signal of the output of the first resonating element. This stage also comprises a second variable summer having L and P inputs and an output, with its output connected to the input of the second resonating element. The P input of this second variable summer is connected to the inverted signal from the inverter and the L input is connected to the output of the second resonating element. The second variable summer is adapted to generate its output at a second phase by combining its L and P inputs.
In another embodiment of the present invention, each stage of the LC ring oscillator is of implemented by a differential LC tank and a variable summer with differential input and differential output signals and current sources.